Metal deposition process



This invention relates to a process for the production ofmetal deposits,and has for its object to produce by .thermal decomposition of metalcompounds completely vuniform fine-grained metal deposits of greatadhering strength on substances of all kinds, that is to say, both oninorganic and on organic substances, regardless of the configuration ofthe surface of those substances. In contrast to known processes, thismetal deposition is effected with great economy. A particular object isto produce -metal deposits on heat-sensitive organic substances of veryfine structure, for example, textile fibres or fabrics. The method isalso useful to metallizethe inner surface ofporous bodies, such as woodor ceramic.

The present application is a continuation-in-part of my copendingapplication Serial No. 190,306, filed October 16, 1950, now Patent No.2,698,812, in which the-claims were restricted to that phase of theinvention relating to alternately heating the surface to be coated anddirecting jets of decomposable metal compound there- .against in rapidsuccession.

The claims of this invention are directed to other methods of effectingthis type of coating as originally disclosed, such as effectingdecomposition of the metal compound at a distance from the surface to becoated but under substantial pressure, the pressure being explosivelyreleased to direct jets of extremely high velocity against the surfacewhereby the mean free path by such great velocity becomes extended.Alternately the metal compound may be coated upon the surface anddecomposed immediately upon the surface.

In the process according to the invention, such metal deposition iseffected by separating the metal from the metal compound at such a shortdistance from the surface of the material to be metallized that theagglomeration (coarsening of the grain) of the metal particles beforethey are deposited is prevented, by maintaining the temperature of thesurface to be metallized at an equal level during the separationprocess, and by limiting the maximum heating of the material to bemetallized preferably to the surface layer.

In order that this process may achieve the desired result, theconditions of dissociation of the metal compound must bemaintained asuniform as possible on or above the deposition surface during theseparation of the metal. The maintenance of constant dissociationconditions in the thermal decomposition of metal compounds is anecessarypre-condition for ensuring a uniform, finegrained and firmly adheringdeposit.

If it is desired to obtain a fine-grained deposit having excellentadhering strength on any desired materials, including more sensitiveorganic substances and on smooth, rough or porous surfaces, or surfacesof any other form, care mustbe taken to compensate for the heat lossesoccurring during the metalizing operation, by heating the surface of thesubstance to be metallized to the decomposition temperature from theoutside and constantly supplying heat from the outside during theseparating 2,822,292 Patented Feb. 4, 1958 ice process. The temperatureof the surface of the material Jous o ina y ci cumstances.benhighe than111616111- ..Wmtureoft decomposi o o th met p und- ,EXperi ne a shotn..t a t sl s .of p rticul rly reat impo t n imthe metall zat on oorgan su stances,

s nce. th s rtho rgani substance i m .g n t ad erin eng of. h ..dor s tey a t y.

duced! I11 o r oma tainoonstan eoomp i gt o ,di.t ,on. is proposed in acordanc w th t inv io in t the. de omp sitions. h

nafter described, to, maintain aminimum path ofmoyement-of the metalatornsor fine-metal particles frornthe point at which theyvareseparated, from themetalcompound, to thepoiut, at which t y ed po ited,hati t tsayrto ensur th ft-h metal atoms or part cles pass, along theshortest path to ,conditions that in the separation of metal from thecompound the danger-exists, even in the neighborhood of the surface, ofthe metal particles agglomerating to form coarser particles before .theyreach the surface. It is therefore desirable either tocause thedecomposition actually to take place, onlyrimmediatelyon the surfaceofthe articles to be metallizedror to ensure, when the s paratio ak s plcenta,vdistanoeefn m h r e, th h metal atomsflor. fin r-par ic es p s rpi y a possible to this surface, ,by superimposing on their freemovement va preferred direction towards .the surface. This is done :bydirecting a.;stre arn,of the metalcompound at great velocity towardsthesurfaceof the article, it being possible for the decompositionto.take place at a more or lessgreat ,distance fromthe surface.

Theworking conditions hereinbeforereferred to which order ,to producecompletely.satisfactorymetal deposits I frommetal mp u n e y, onstnthrep oduoible reaction conditionsat thepoint ofseparation, heating atconstant temperature preferablypnly atthe surface of the article to bemetal1ized, -,a nd prevention of ,agglomeration of the metal-atoms orvery fine particles separated from the metalcompounds byadjusting-ithelength o t r t e m v me t, co s itute-incombin ationnew workingdirections.

If the d o tio takes p ac .direot yeon 1 36x5131- face of the article tobe metallized-pr t adistanoe; therefrom, it is in all circumstancesirnportant thatonly "the surface itself, but not ,the r nass pf-theentire article, .-.be heated because he t n of th -mass ft o riol i comply unimportant :t th m ll zin :PB QQ and 1-involves a considerableglossof heat. Moreespecially if only. the surfaceonwhich the metal compound;is decomposed is heated, the conditions of dissociation-canine .muchmore readily maintained uniform andgthe metallizing process canproceedat greatspeed overvthewsurface. Thus, only a fraction of thethermaLenergy-ctherwise necessary for heating the entire article tothe;decomposing temperature is required for the metallizaiion.Similarly, the time taken by the metallization ;is only a fraction ofthat taken in the known processes.

The heating ofvthesurface tothedecomposition temperaturedoesnot.preclude-pre-heating ofthe ,ar-ticle'below the decompositiontemperature from being expedient in special cases.

Reference is made to .my; parent application, now {Patent ;2,69.8;81-2,; da'ted .lanuaryr4, 195.5,forfurther detailed description ofthe subject matter there claimed. The subject matter claimed in thisapplication will be understood by reference to the single figure of thedrawmg.

In the metallization of materials or articles by heating and depositiondirectly on the surface, the metallization may be eifected in vacuo inspecial cases, for example in the metallization of very smooth surfaces,one of the methods already described being applied. Generally, however,the new metallizing process will be most economically carried out atatmospheric pressure.

While the methods claimed in my parent application of which this is acontinuation-in-part are described in the foregoing examples whichefiiect fundamentally a heating and deposition by jets on the heatedsurface of the article to be metallized, the process according to theinvention and as herein claimed is carried out, as illustrated by Way ofexample in the single figure comprising the drawing herein by effectingthe decomposition of the metal compound, not on the surface, but at adistance a therefrom. In order to prevent the agglomeration of the metalatoms or particles in the movement along the path a to the surface,these metal particles must be blown on to the surface at great speed,for example at a few 100 m. per second. The order of magnitude of thedistance a is in this case of a few millimeters.

The metal compound, for example nickel carbonyl, is under high pressure(about 100 atmospheres absolute pressure) in a pressure-proof container31 which is brought by means of an electric heating element 32 to atemperature (for example, 240 C.) lying above the decompositiontemperature of the metal compound at normal pressure. The metal compoundis blown at high speed on to the surface of the article 1 through anoutlet aperture 34 adapted to be closed by a cone valve 33 or the like.The expansion occurring at the discharge of the metal compound bringsthe metal compound out of the stable condition into the unstablecondition, so that it is decomposed.

In the process according to the invention herein claimed, it will beseen that a metal compound is brought to a high temperature in apressure vessel and stable in vapor form at the high pressure and isthen decomposed by relief of pressure. There is thereby obtained anappreciable simplification of the metallizing process, since the heatingof the body to be metallized is wholly obviated, and accordingly thereare avoided all those difficulties of temperature control which areassociated with the thermal decomposition of metal compounds. In thisprocess the decomposition can take place under unvarying conditions, i.e., with the temperature and pressure kept constant during the relief ofpressure, so that therefore bodies of all kinds and having surfaces ofany shape can be given a metal coating which is completely uniform, isof very fine grain and adheres extremely well. Since with the presentprocess the body to be metallized does not require to be heated, it ispossible to employ this process for metallizing heat-sensitive organicsubstances, for example paper, textiles and the like.

Metal carbonyls, for example nickel or iron carbonyl, are particularlysuitable for use in carrying out the present invention. When usingnickel carbonyl it is brought, in a pressure-tight container, to apressure of for example about 100 atmospheres and a temperature of about240 0., this temperature being above the decomposition temperature ofthe metal compound at normal pressure.

The metal compound is allowed to issue from this container through anozzle. The pressure relief, to a low pressure, which occurs during theemergence brings the metal compound from the stable condition to theunstable condition, so that it is completely decomposed. The

' metal compound, or the products of decomposition, are -blown at highspeedon to the surface of the body to be metallized, for example at morethan meters per second, by the high pressure existing in the container.

Care must be taken that the relief of pressure, or decomposition, of themetal compound takes place at so short a distance from the surface to bemetallized that aggregation of the metal atoms and particles isprevented and at the same time a very firmly adhering, extremely finegrained metal coating is obtained. This distance amounts to a fewmillimeters, having regard to the high blowing speed.

In carrying out the process according to the invention, all metalcompounds may be employed which are dissociated into metal and residualsubstance under the action of heat. Such compounds are mainly metalcarbonyls, metallic hydrides and metallic halogens. Examples of metalcarbonyls are nickel tetracarbonyl, iron pentacarbonyl and the carbonylsof tungsten, molybdenum and cobalt. The hydrides which may be employedinclude, for example, copper hydride, germanium hydride, antimonyhydride and others, while the chlorides include chromium chloride andnickel chloride, and the other types of compounds which may be employedinclude particularly metallic acetyl acetonates such as copper acetylacetonate. In addition to these compounds which can be directlyseparated under heat, metal compounds which liberate the metal onreduction or chemical reaction by thermal means may also be employed forthe metallizing process according to the invention. As examples for thereduction method may be mentioned the oxides of the precious metals andof other heavy metals, and for the chemical reaction method the halogencompounds of the heavy metals, for example chromium chloride, areparticularly suitable.

The process according to the invention may be employed inter alia toproduce metal deposits on compact articles or on and in porous articlesor materials. One of the characteristic features of this process residesin the fact that the quantity of metal deposited per unit time issubstantially greater than in the metal vaporization process and is evengreater than in the galvanic process. Moreover, metallic deposits can beapplied by means of the process to bright, oxidized or otherwise coatedmetals, and especially to non-metals such as plastics, wood, fabrics,papers of all kinds, felts, fibres of organic and inorganic nature,glass, quartz, ceramics, salts and other compounds; that is to say, toall solid or plastic materials whose surface does not undergo anydetrimental physical or chemical modification during the metallizingoperation.

Extremely fine projections or pores can be filled by the metal deposit.Sensitive organic substances are also not damaged if the metallizationprocess is correctly conducted. For example, papers, textiles and thelike may be coated on their outer and inner surfaces with a metal filmwhich embraces each individual fiber and which, if of suitablethickness, cannot be mechanically detached without partial destructionof the fibers.

The metallization is so clean and true to the surface and adheres sostrongly that it can be used as a support for a further galvanicdeposition of any desired metals. For this purpose, only an extremelythin layer deposited by the process described is required in the case ofmetallized non-conductors.

Naturally, the conductive layers deposited from the gas phase have, ascompared with the layers hitherto produced by the deposition of silveror by the application of layers of graphite or silver sulphide coatingsby the wet method, an immeasurably greater adhering strength which canonly be compared to the ideal attachment of fine grained deposits by thecathode atomization or vaporization process. Since the economy of theprocesses for the separation of metals from compounds according to theinvention is nevertheless considerably greater and it must be taken intoaccount that the adhesive strength of a conductive layer on anon-conductor is essential to the adherence of the metal filmsubsequently galvanically deposited, the process is also of particularimportance in the electroplating field.

I claim:

1. Process of depositing a uniform firmly adherent coating of metallicparticles of finely divided sub-crystalline size on the surface of abase material, comprising heating a heat decomposable metal compound toa sub stantially constant temperature exceeding the decompositiontemperature of said compound at normal pressures in a confining vesselwhereby to develop high pressure substantially exceeding atmosphericpressure and confining the heated compound under sufiicient pressure toprevent its decomposition at the elevated temperature, and thenexplosively releasing the confined vapors in a jet directed against thesurface to be coated, the expansive release bringing the metal compoundout of the stable condition into the unstable condition, whereby it isdecomposed at the discharge of said vessel.

2. Process as defined in claim 1, wherein the heat dccomposable metalcompound is heated to a temperature exceeding the decompositiontemperature at normal pressures suflicient to develop a pressure of theorder of 100 atmospheres prior to release of pressure in a jet againstthe surface to be coated.

3. Process as defined in claim 1, wherein the heat decomposable metalcompound is a metal carbonyl.

4. Process as defined in claim 1, wherein the surface with which themetal is coated is composed of inorganic material.

5. Process as defined in claim 1, wherein the surface with which themetal is coated is composed of organic material.

6. Process as defined in claim 1, wherein the surface with which themetal is coated is composed of a thin heat decomposable sheet of organicmaterial.

7. Process for depositing a uniform firmly adherent coating of metallicparticles of finely divided subcrystalline size on the surface of a basematerial, comprising heating a thermally decomposable metal compoundconfined in a container to a temperature exceeding its decompositionpoint and to develop a pressure exceeding about 100 atmospheres, saidpressure serving to maintain the compound stable, and then decomposingthe compound by relieving the pressure in said container, whereby saidmetal compound decomposes and is emitted from said container as a highvelocity jet, and applying said jet against successive small incrementsof surface area of the surface to be coated, while regulating thedistance of travel of said jet from the point of release of saidcontainer to the surface being coated to less than the mean free path ofthe fine metal particles separated in the decomposition of the metalcompound.

8. A coated product having a firmly adherent coating of finely dividedparticles of metal of subcrystalline size deposited thereon by impingingthe jet thereagainst resulting from explosive release of a heatdecomposable metal compound heated to a temperature exceeding itsdecomposition point at atmospheric pressure to a substantially highpressure and held at a distance from the surface to be coated wherein nometal dust is released.

9. A coated product having :a firmly adherent coating of finely dividedparticles of metal of sub-crystalline size deposited thereon bydecomposing a heat decomposable metallic compound to finely dividedmetal and applying said finely divided metal to the surface to be coatedimmediately before the metallic particles have had opportunity tocoalesce into larger particles.

References Cited in the file of this patent UNITED STATES PATENTS1,128,059 Schoop Feb. 9, 1915 2,631,948 Belitz et a1. Mar. 17, 19532,698,812 Schladitz Jan. 4, 1955 2,701,775 Brennan Feb. 8, 1955

1. PROCESS OF DEPOSITING A UNIFORM FIRMLY ADHERENT COATING OF METALLICPARTICLES OF FINELY DIVIDED SUB-CRYSTALLINE SIZE ON THE SURFACE OF ABASE MATERIAL, COMPRISING HEATING A HEAT DECOMPOSABLE METAL COMPOUND TOA SUBSTANTIALLY CONSTANT TEMPERATURE EXCEEDING THE DECOMPOSITIONTEMPERATURE OF SAID COMPOUND AT NORMAL PRESSURES IN A CONFINING VESSELWHEREBY TO DEVELOP HIGH PRESSURE SUBSTANTIALLY EXCEEDING ATMOSPHERICPRESSURE AND CONFINING THE HEATED COMPOUND UNDER SUFFICIENT PRESSURE TOPREVENT ITS DECOMPOSITION AT THE ELEVATED TEMPERATURE, AND THENEXPLOSIVELY RELEASING THE CONFINED VAPORS IN A JET DIRECTED AGAINST THESURFACE TO BE COATED, THE EXPANSIVE RELEASE BRINGING THE METAL COMPOUNDOUT OF THE STABLE CONDITION INTO THE UNSTABLE CONDITION, WHEREBY IT ISDECOMPOSED AT THE DISCHARGE OF SAID VESSEL.